Connecting a computer to the Internet and other public networks is an increasingly popular way for home computer users to receive multimedia information and communicate with other computer users. As a result, higher and higher data bandwidth is needed by home computer users. Many efforts are being made to use the already existing communication infrastucture to provide high-speed access to network services. The plurality of existing digital TV Hybrid Fiber/Coaxial ("HFC") cable networks is one of the most suitable communication mediums available to home users for connecting to the Internet.
An HFC headend is the originating point of a signal in an HFC cable network, and it is also responsible for maintaining synchronization among the users to ensure that they do not create contentions in the distributed system when attempting to send data to the headend via a shared transmission medium. The synchronization is difficult because it must account for variable and differing data propagation delays.
FIG. 1 is a block diagram that illustrates a typical headend for providing Internet access to a plurality of user computers, referred to as "client computers". In FIG. 1, a headend 10 is coupled to an Internet access point 12. Headend 10 transmits a packet data stream 32 over a transmission medium 30 to client computers 21-24. In the embodiment shown in FIG. 1, headend 10 is adapted for a HFC cable system, so transmission medium 30 is an optical and coaxial cable, and client computers 21-24 each include cable modems for transmitting and receiving a data stream.
Headend 10 transmits (broadcasts) payload and auxiliary data streams which can be received by the plurality of client computers 21-24. To provide an upstream channel (i.e, a channel that permits data to be transmitted from client computers 21-24 to headend 10) headend 10 also remotely controls how client computers 21-24 access and transmit on transmission medium 30. Typically, Time Division Multiple Access ("TDMA") is used to provide the upstream channel.
With TDMA, each client computer transmits to headend 10 only during a specific time interval called a time slot. Client computers 21-24 keep track of time slot synchronization by maintaining and adjusting their internal clock reference, based on Media Access Control ("MAC") information broadcast by headend 10. The time slots are assigned to the client computers by a MAC function within headend 10.
As a result, client computer slot markers are synchronized to headend 10's master slot markers, which are based on a common system clock provided by the MAC function. Headend 10 transmits information about the nominal position and current sequential number of the slot markers in a special packet called a Timing Information Element ("TIE") packet.
When received by client computers 21-24 the TIE packet provides a time stamp of when it was generated based on the system clock. Each client computer 21-24 can synchronize its own slot marker from the information in the TIE packet.
Headend 10, which is a "data-only" headend, receives network data from a network data source 14, modulates a radio frequency (RF) carrier with a modulator 34, and transmits the resultant signal to the transmission medium 30. The network data, which is generated by Internet 12, is then received by client computers 21-24.
Network data source 14 generates packet data stream 32 and includes the system clock 20, a TIE packet generator 18 that generates TIE packets, and a data generator 16 that formats data packets received from Internet 12. Modulator 34 converts packet stream 32 generated by network data source 14 into the appropriate form (e.g., radio frequency) so that it can be transmitted on transmission medium 30.
For the upstream channel, additional functions performed by network data source 14 include upstream signal demodulation and reformatting data into a stream suitable for forwarding to Internet 12. The receive side of headend 10, which receives data streams transmitted from client computers 21-24, is not shown in FIG. 1.
When a TIE packet is transmitted to client computers 21-24 from headend 10, the TIE packet arrives at different times to each client computer 21-24 due to different propagation delays among the client computers. For example, client computer 24 will receive a TIE packet at a later time than client computer 21 because it is located at a further distance from headend 10. However, the delay to each client computer 21-24 is fixed because the delay due to modulator 34, transmission medium 30, and each client computer 21-24 does not change. Therefore, the delay can be measured and factored.
The delay can be determined by each client computer by comparing the TIE packet with a synchronization symbol sent from headend 10 to all client computers. The client computers then compensate for the delay when performing slot marker synchronization. A known procedure, referred to as "ranging", is used to adjust the synchronization in client computers 21-24 to compensate for the constant delay of any origin.
FIG. 2 is a block diagram that illustrates a typical multi-purpose headend. In a multi-purpose headend, the data service and regular audio and video services may coexist on the same frequency channel if audio and video services are digital (e.g., as defined by the Digital Video Broadcaster's consortium ("DVB" under European Telecommunication Standards Institute ETS 300 429)) This allows bandwidth to be used economically and provides rich content services to subscribers (client computers). The network data stream may be either used to enhance the video stream (i.e., to provide additional on-screen information, to provide a database that may be accessed while watching sports, etc.), or used independently.
Multi-purpose headend 50 shown in FIG. 2 includes a video data source 40 that generates a video data stream and an audio data source 42 that generates an audio data stream. The data streams generated by video data source 40 and audio data source 42 typically are transmitted on the same transmission medium 30 as the data stream generated by network data source 14. One example of headend 50 is a headend that provides cable television and Internet access over the same coaxial cable.
In order to combine the three data streams generated by headend 50, the data streams are multiplexed together by a multiplexer 46 before being sent to modulator 34. However, multiplexer 46 multiplexes input data streams on a packet-by-packet basis and has to account for potentially variable data rates for each data source. Therefore, multiplexer 46 introduces a variable (and in most cases unknown) propagation delay due to the internal data buffering required to synchronize incoming data streams. The time delay for the TIE packets generated by network data source 14 to reach the client computers is no longer a fixed delay as it is for headend 10 of FIG. 1. Therefore, the ranging procedure cannot properly compensate for the delay when performing slot marker synchronization, and headend 50 cannot maintain the proper TDMA synchronization with the client computers.
Based on the foregoing, there is a need for a method and apparatus for allowing a headend which transmits a TDMA-based data stream multiplexed with other data streams to maintain TDMA synchronization with client computers.